In order to evaluate the immunoprotective effect of Tol C (Genbank: EPB97112.1), a recombinant Tol C domain protein named B6 was obtained by prokaryotic expression. The immunogenicity of B6 was preliminarily detected by rabbit anti-Pseudomonas plecoglossicida polyclonal antiserum. Followed, a subunit vaccine was prepared by mixing B6 with alum adjuvant, and the large yellow croaker was immunized by intraperitoneal injection. The relative protection rate (RPS) was detected 4 weeks after immunization. Meanwhile, the serum, head kidney and spleen of fish were collected at 2,4,6 and 8 weeks after immunization, and the serum antibody titer as well as the expression level of immune-related genes were determined. Finally, the head kidney and spleen of healthy individuals and challenged ones were both collected and made into paraffin sections to compare histopathological changes. The results showed that: a recombinant Tol C functional domain protein named B6 was obtained, with the molecular weight of 22 kD. Western Blot results showed that the rabbit antiserum against Pseudomonas plecoglossicida could strongly bind to B6 with a single special band. 4 weeks after inoculation with B6 subunit vaccine, the RPS reached 72.22 %. ELISA showed that the serum antibody titer increased significantly since the 2nd week after immunization, peaked at the 4th week (log2 8.65), and then decreased slightly to log2 6.98 at the 8th week. RT-PCR showed that the expression level of immune-related genes in head kidney and spleen were both up-regulated with varying degrees at 2, 4, 6 and 8 weeks after immunization. Among them, the up-regulation rates of IL-1β and MHCIα was the largest, with 93 and 77 times each in head kidney, and 241 and 131 times each in spleen, respectively. Except for CD8, the expression up-regulation trend of IgM, IL-1β, CD4, MHCIα and MHCIIβ in spleen was all higher than that in head kidney. Paraffin sections showed that severe bleeding spots and nodules appeared in the head kidney and spleen of the control group after challenge, accompanied by different degrees of siderophore deposition. Comparatively, there were only a few bleeding spots in the head kidney and spleen of the B6 immunized group, and no nodules and siderophore deposition. In conclusion, our study first reported the prokaryotic expression and immune protection evaluation of the outer membrane protein Tol C from Pseudomonas plecoglossicida, the results showed the recombinant Tol C protein had strong immunogenicity and good immune protection in large yellow croaker, with a RPS of 72.22 % 4 weeks after immunization, thus is a potential candidate of subunit vaccine.